The invention relates to a method for producing calendered paper and an apparatus for implementing the aforementioned method.
After the drying of paper, the desired surface structure of the web is produced by means of a mechanical treatment of the surface of paper, i.e. calendering. There are many calendering methods, but it is characteristic to all of them that the web is led through one or several nips, which is/are formed between two rotating rolls that are pressed against each other. The purpose of the calendering is to improve the properties of a paper web, especially its thickness profile, smoothness, gloss as well as porosity and opacity of the surface. The formability of the paper web in the nip is affected by the temperature of the calender rolls, the nip pressure prevailing in the nip and the moisture content of the web. By adjusting these parameters it is possible to affect the above-mentioned physical properties of the paper web.
At present, the end users of paper demand paper grades of higher quality. The speeds of paper machines have also increased, which has resulted in rapid development of on-line calendering methods. However, on-line calendering methods are not suitable for production of high quality printing paper grades with especially high surface quality requirements, such as SC grades. These high quality paper grades must still be produced by using re-reeling and off-line multiroll or super calenders after the drying of the fiber web, wherein several, even three or four such calenders are used in parallel in one paper machine to satisfy the production capacity.
Supercalenders are calendering units in which nips are formed between a press roll having a smooth surface, such as a metal roll, and a roll provided with a flexible coating, such as a paper or polymer roll. Supercalenders typically comprise 10 to 12 nips of which one is a so-called reversing nip in which two rolls having a flexible surface are positioned against each other. By means of the reversing nip it is possible to treat both sides of the web to make them substantially similar. The supercalender rolls are conventionally arranged on top of each other, wherein they form a high stack of rolls.
Conventionally, SC grades are manufactured in such a manner that the paper is dried, “overdried” to a moisture content of approximately 2 to 4% in the drying section of the paper machine. Thereafter the paper is moistened to calendering moisture of 8 to 11% before reeling up. On the reel, the moisture of the paper has time to stabilize in the z-direction of the paper before supercalendering by means of the supercalender.
However, the moistening of paper to the above-mentioned moisture content causes problems. It is difficult to reel up such paper having a moisture content of 8 to 11% and to handle the paper reels as well. Problems are also caused by the shrinking of the web in the CD and/or MD direction resulting from the drying of the paper before calendering. The height of the stack of rolls in the supercalender sometimes also causes problems relating to space in a paper mill.
Normally, when paper is calendered the smoothening and glazing of its surface takes place at the expense of the thickness of paper. In the calender, linear load and/or nip pressure are/is used as a control variable, which determines the surface quality of the paper as well as the final thickness. In other words, when the aim is to attain good surface quality, the thickness of paper is reduced. Attempts have been made to solve this problem by means of so-called gradient calendering, in which paper is moistened right before calendering in such a manner that the moistening water has time to be absorbed only in the surface of the paper web, wherein the effect of calendering is exerted only on the surface layers of the web, and the calendering does not bring about changes in the density of the central parts of the web or a substantial change in the thickness of the web.
Gradient calendering has been applied in on-line calenders in which the paper web is guided directly without re-reeling from the drying section of the paper machine to a calender containing several nips, for example to a supercalender. Such a method is disclosed for example in U.S. Pat. No. 6,401,355, in which the web is overdried in the drying section of the paper machine and moistened again before it is guided to an on-line calender located in connection with the paper machine. The amount of moistening water and the absorption time of the water is selected in such a manner that the moistening water only affects the surface layers of the web. In U.S. Pat. No. 6,758,135 one side of the paper web is moistened with a first moisturizer before the web is guided to a supercalender, and the opposite side is moistened with a second moisturizer after the reversing nip of the supercalender. U.S. Pat. No. 6,698,342 also discloses the use of two moisturizers in connection with the calendering process, in which a multiroll calender is divided into two roll sets of rolls and a first moisturizer is positioned before the calender and a second, a so-called intermediate moisturizer is positioned between the set of rolls.
However, by means of the methods according to these publications it has not been possible to attain the high surface quality required of SC papers. Furthermore, the methods presented by these publications are only suitable for on-line calenders. In practice, moisture gradient calendering has not yet been applied in off-line multinip calenders.